A calorie is a measure of the heat required to raise the temperature of water 1 degree Celsius. As it pertains to food intake, calories can simply be thought of as a measure of the energy a particular food provides to the body. Several factors determine the amount of calories the body requires to perform basic daily functions, some variable and some fixed.
Basal Metabolic Rate
Your basal metabolic rate, often just referred to as metabolism, is the energy your body requires to perform basic functions such as digestion and respiration. People with higher basal metabolic rates will burn calories more quickly. Your basal metabolic rate is influenced largely by genetics but can also fluctuate with activity level and the type of food you eat. In general, more activity and smaller, more frequent meals will lead to a higher basal metabolic rate and more caloric expenditure.
Activity
Higher levels of activity result in greater energy expenditure and a greater need for caloric intake. The amount of calories burned during a particular activity depends on that activity's metabolic equivalent, a measure of the amount of oxygen required to fuel the body. Activities that are more physically exerting have greater metabolic equivalents and expend more of the body's energy supplies.
Age
As you get older, your basal metabolic rate tends to decrease, resulting in a reduced daily caloric requirement. However, you can keep your basal metabolic rate high by remaining active. The American College of Sports Medicine recommends getting at least 30 minutes of activity, five days a week to maintain general fitness and a high basal metabolic rate.
Sex
In general, men have more muscle and less body fat than women, both of which lead to a faster metabolism. Furthermore, men usually have larger bodies leading to a greater overall caloric need. Adult women typically need about 2/3 the daily caloric intake as men, according to the National Strength and Conditioning Association.
Weight and Body Composition
People with larger bodies need more calories to maintain their weight. Furthermore, larger bodies have more muscle content and burn calories faster, the Mayo Clinic reports. The amount of muscle you have versus fat -- your body composition -- greatly influences your caloric need. Muscles use up calories more quickly than fat, and people with higher muscle content tend to have higher basal metabolic rates as well.
Goals
Your weight and body composition goals have a great effect on your caloric needs. To lose weight, you need to obtain a negative energy balance -- you need to expend more calories than you consume through food. It takes about a 3,500-calorie deficit to lose 1 lb. of fat, according to the National Strength and Conditioning Association.
References
- "Physiology of Sports and Exercise"; Jack H. Wilmore and David L. Costill; 2004
- "Resources for the Personal Trainer"; American College of Sports Medicine; 2010
- "Essentials of Strength Training and Conditioning"; Thomas R. Baechle and Roger W. Earle (eds.); 2008
- Mayo Clinic: Metabolism and Weight Loss: How You Burn Calories
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